Ion mobility spectrometers (IMS) measure mobility of ions in gases influenced by an electric drawing field. The mobility of an ion can provide information on its size and shape. Mobility spectrum can identify the ionized substances in the spectrometers such that trace pollutants, for example, can be measured.
During operation of a typical ion mobility spectrometer, ions are continuously generated in short pulses by an ion source and introduced into a drift region of the spectrometer via a gating grid. The gating grid can be a fine bipolar grid such as a Bradbury-Nielsen grid. The introduction pulse periods usually last between 150 and 300 microseconds, and the recording of the spectrum takes about 30 milliseconds. The ion packets transmitted through the grid by the pulses are pulled through a collision gas in the drift region by an axial electric field. The measured mobility of the ions is used to determine velocities of the ions, which, as known in the art, depend on their collision cross-section, their mass, their charge, their ability to become polarized and their tendency to form complex ions with molecules from the collision gas.
Ion species such as monomers, dimers, trimers, doubly charged monomers and complexes with water and collision gas molecules are typically formed in the ion source from molecules of a gas (e.g., air) introduced into the spectrometer. Every ion species has a characteristic mobility. At the end of the drift region, incident ion current is measured with the ion detector and digitized as a drift time spectrum or “mobility spectrum”. The mobility spectrum is then stored as a digitized sequence of measured ion current values. The mobility spectra are typically summed over one or even several seconds to determine a sum spectrum with relatively little noise. The summation period can also be shorter than one second where, for example, the dynamic behavior of chemical reactions under changing concentrations is being detected.
Evaluating the mobility spectrum can provide information on the mobility of the ions as well as information on the formation of dimers and doubly charged ions. This information can in turn indicate the identity of the substances in the gas introduced into the spectrometer. Additional information regarding the identity of the substances can be determined by acquiring a series of mobility spectra under variation of the concentration of the substance, as they occur naturally when substances appear or vanish in the air. Changes of the signal intensity ratios of dimer ions to monomer ions or doubly to singly charged ions can provide additional information regarding the identity of the substances and substance concentrations within the gas.
An ion mobility spectrometer may be used to detect, for example, explosives or drugs in suitcases/baggage at an airport. In this example, the suitcases are swabbed. The swab spot on the swab material is heated close to the inlet of the mobility spectrometer such that a cloud of vaporized substance enters the ion source of the spectrometer. The concentration increases rapidly in about a second and drops again in five to ten seconds. Initially the signal of the monomer ions increases, but is soon overtaken by that of the dimer ions, after which trimer ions can appear. The temporal behavior of the ion ratios is characteristic of the vaporized substances and provides additional identification information, which can reduce the rate of false alarms (i.e., false positives). Approximately five to ten mobility spectra per second must be acquired to measure the dynamic characteristics, which requires a method with a relatively high signal-to-noise ratio.
Approximately between one half a percent and one percent of the ions of the gaseous substance introduced into the spectrometer are utilized for a conventional spectrum measurement repetition rate of about 30 spectra per second, and an ion transmission time of between 150 and 300 microseconds. The remaining ions are discharged, for example at the gating grid, and are thereby lost to the measurement process. Notably, by increasing the percentage of the ions utilized from one percent to 50 percent, the signal-to-noise ratio can be improved by a factor of √(50)≈7.
U.S. Patent Application Publication No. 2009/0236514 A1 to Uwe Renner, which is hereby incorporated by reference, discloses a method for analog modulating the ion current from the ion source with a continuous modulation function. The continuous modulation function has an instantaneous frequency varying over a wide frequency range. The resulting ion current signal at the detector is decoded by a correlation with the modulation function, which provides a relatively noise-free mobility spectrum with relatively good mobility resolution. A “chirp” is preferably used as the modulation function. The chirp is a sine wave type function, for example, whose instantaneous frequency increases from zero to an upper limit of a few kilohertz.
The modulation frequency is preferably varied as a single chirp that is extended over the chosen measuring time T. The chirp frequency is varied from a lower frequency limit of zero hertz (Hz) to an upper frequency limit v. The upper frequency limit determines the maximum mobility resolution. An upper frequency limit of seven kilohertz, for example, provides peak widths at one half the maximum height of about 200 microseconds. Preferably a “linear chirp” is used whose frequency increases linearly in time t. Such a linear chirp has the form f(t)=a+b sin(πv t2/T), where 0≦t≦T.
The modulation control signal for the gating grid is generated by a digital-to-analog (D/A) conversion of previously calculated and stored values of the modulation function. The conversion rates of the digital-to-analog conversion of the modulation control signal, and also the analog-to-digital conversion of the ion current from the detector, should be relatively fast (e.g., at least five times the upper frequency limit). The bandwidth of the amplification and the bit resolution for the digital conversion should be higher than the bandwidth for a pulse-operated ion mobility spectrometer, because the ion currents with different frequency components can be superimposed on one another. The bandwidth, however, should not reach the saturation limit of the electronics.
On average, 50% of the ions are transmitted where the modulation has symmetrical control. Substantially all of these transmitted ions contribute to the generation of the mobility spectrum at full modulation depth between zero and one hundred percent of the ion current. Fifty percent of the ions therefore are utilized during the measurement. The variation of the modulation frequency in the chirp preferably starts at zero hertz and ranges up to about seven kilohertz for a typical spectrometer with a 10 centimeter drift length. This modulation affects all ion species. The patterns applied to the individual ion species move along with different drift speeds as the ions drift through the drift tube of the mobility spectrometer, such that the ion current at the ion detector exhibits a complicated pattern of superimpositions. The temporal sequence of the ion current is measured at the end of the drift region, digitized and stored. The stored signal pattern is decoded by a correlation with the modulation function, and the mobility spectrum of the ions therefore can be determined. The measurement is synchronized in time with the output of the modulation control signal.
The afore-described method has a tendency to generate single side bands of intense mobility signals in the mobility spectrum when typical gating grids are used as the modulators. In particular, the shorter the measuring time T chosen for a mobility spectrum, the stronger the formation of the side bands. Experiments and simulations have shown that these side bands are generated by a weak nonlinear modulation characteristic of the gating grid used as the modulator. The term “nonlinear” relates to the modulation of the ion current amplitude, and not to the increase in frequency of the chirp.
Disadvantageously, the side bands can imitate mobility signals of other ions. The side bands imitate signals of substances with low concentration, although they are broader than normal mobility signals. They complicate the evaluation of the mobility spectra, particularly when the dynamic characteristic is also to be measured by using short spectrum acquisition times.
FIG. 2 illustrates the measured transmission characteristic of the gating grid for the ion current in picoamperes (ordinate) as a function of the control voltage in volts (abscissa). The transmission characteristic has a broad linear operating range which appears, at first glance, suitable for a modulation. A more detailed analysis, however, shows a weak nonlinearity in the apparently linear region, which is shown by the curves of the two saturation regions merging slightly differently into the central linear region of the curve. This weak nonlinearity, which can be shown in mathematical simulations, can create side bands having strong mobility signals. These slight nonlinearities can also be created in spectrometers that use other types of grids.
What is needed therefore is an ion mobility spectrometer that can compensate for nonlinearities.